Guide · NEC 2023
Ampacity vs voltage drop
Two independent checks. The wire must pass both. Short runs are ampacity-governed; long runs are drop-governed.
1 · Ampacity = heat (mandatory)
Table 310.16 limits current so insulation stays below 60/75/90°C. Derate for heat (ambient) and bundling, cap at terminals per 110.14(C), cap breakers per 240.4(D). This is Code — inspectors enforce it.
2 · Voltage drop = performance (recommended)
NEC informational notes recommend ≤3% branch, ≤5% total (feeder + branch). Not Code-enforceable in most jurisdictions, but motors stall, lights dim, and electronics fault when drop is high. Formula: 2·K·I·L/CM single-phase (K=12.9 Cu, 21.2 Al, L = one-way feet).
Rule of thumb
- 120V 20A on #12 Cu: ampacity passes, but drop exceeds 3% past ~60ft one-way → upsize to #10.
- 240V loads tolerate 2× the distance of 120V at the same drop %.
- Three-phase drop is lower per amp than single-phase for the same distance.
- Upsizing one size typically halves the “over-drop” on marginal runs and future-proofs the circuit.